1. Field of the Invention
This invention pertains generally to data multiplexers and more specifically to a Sagnac interferometer amplitude modulator based demultiplexer for high-speed return-to-zero optical time division multiplexed data.
2. Description of the Related Art
Due to the speed limitations of electronic circuitry, future time division multiplexed optical communications networks operating at data rates in the order of 100 Gbits/s will require optical demultiplexing techniques. Toward this end, a variety of all-optical demultiplexing schemes, based on third order nonlinearities in fiber or on resonant nonlinearities in semiconductors, have been proposed and demonstrated. These require a local optical clock to drive the demultiplexing; furthermre, many (those based on nonlinearities in fiber) suffer from a long latency between receipt and decoding of the data.
Modulator based demultiplexing schemes have been previously proposed, and demonstrated with speeds up to 40 Gbit/s. These have depended on Mach-Zehnder lithium niobate intensity modulators, which are well known to suffer from bias instability with changes in environmental conditions and applied voltage.
The standard integrated optic intensity modulator consists of a channel microwave Mach-Zehnder interferometer fabricated on a lithium niobate substrate. The Mach-Zehnder interferometer consists of an input waveguide which branches to a pair of parallel optical waveguides (a"Y-branch"), at least one of which is positioned under an electrode so that its optical path length may be varied by an applied voltage, via the electro-optic effect. The two waveguides are combined at the output(at a second Y-branch, oriented to combine the light in the two waveguides), where the light interferes according to the differential optical phase imposed by the applied voltage. However, the tolerances involved in the fabrication of the input and output Y-branches restrict the ON/OFF extinction ratio to .about.20 dB. This is due both to the splitting ratio between the two arms being other than 50%, and to the fact that the splitting ratios of the two branches are typically unequal.
The problem of bias drift is ubiquitous with Mach-Zehnder modulators. The phenomenon stems from fundamental material properties of the LiNbO.sub.3 substrate, in combination with the topology of the Mach-Zehnder interferometer. The transfer function of a Mach-Zehnder interferometer (or indeed any interferometer) depends on the optical path length difference (differential phase) between its two arms as a squared-sinusoid. Since the two optical paths in the Mach-Zehnder interferometer are independent, the differential phase between them in the absence of an applied voltage is arbitrary. For effective modulation, a specific initial phase is required; in Mach-Zehnder devices, this is accomplished by applying a direct current (DC), or bias, voltage in addition to the alternating current (AC) modulation. However, the LiNbO.sub.3 substrate is a piezoelectric, electro-optical material, with a small but significant electrical conductivity. Thus, the substrate transduces the applied bias voltage and changes in a variety of environmental conditions (including temperature, mechanical stress, humidity) into an optical path length difference between the two interferometer arms which varies in time. This is the bias drift which must be tracked with compensating electronic feedback to the applied DC voltage for the modulator to operate at a fixed bias phase.
Optical time division multiplexing (OTDM) is a technique for transmitting data over optical fibers at very high data rates (e.g., 100 Gbit/s) by temporally interleaving multiple slower rate (e.g., 10 Gbit/s) data streams. This technique allows access to the virtually unlimited bandwidth of the optical fiber transmission medium, despite the fact that the electronics used to generate and encode the data are relatively narrowband.
The multiplexing (MUX) process is the conversion of relatively slow speed parallel data streams to a single very high speed serial stream. While it is relatively trivial to construct a high speed OTDM data stream, decoding that data once it reaches its destination is much more difficult. The data speed is well beyond the capabilities of electronics, either at present or in the foreseeable future. In order to decode the data, some means is required for extracting a single low speed data channel from the high speed stream, or, more generally, for converting the high speed serial data back into the several lower speed parallel channels. This process is known as demultiplexing (DMUX). Over the past few years, a number of schemes have been demonstrated capable of demuliplexing 100 Gbit/s data. These have universally relied on using some optical correlation scheme, wherein a local optical clock signal is combined with the incoming optical data stream in a nonlinear optical medium. Examples include four-wave mixing and cross-phase modulation in optical fiber and semiconductor laser amplifiers.
All of these techniques rely on an optical clock signal, consisting of a high peak power pulsed laser with a repetition rate equal to the data rate of the individual channels. Generating this optical clock stream adds a great deal of complexity and expense to the demultiplexing process. Furthermore, the previously demonstrated schemes are capable of demultiplexing only a single data channel, so that as many demultiplexers as there are TDM data channels would be required to decode the entire data stream. The nature of the optical nonlinearities involved normally dictate that the optical clock consist of a stream of short (e.g., 1-3 ps), high peak power (1-10 W) pulses of light. An example of such a demultiplexer is the polarization multiplexed nonlinear optical loop mirror (NOLM DEMUX) developed by the Naval Research Laboratory. See, Dennis et al., SOLITON LOOP MIRROR DEMULTIPLEXER USING ORTHOGONALLY POLARIZED SIGNAL AND CONTROL, Photon. Technol. Lett Vol. 8, No.7, pp. 906-908, 1996. The optical nonlinearity is the Kerr effect (cross phase modulation) in optical fiber, which is extremely weak. The optical clock in this device consists of an actively mode locked fiber laser using a high power optical amplifier with feedback stabilization to maintain suitable operation. This clock is extremely expensive and would be prohibitive if such a device were required at every node in a network where demultiplexing is required.. The weak nonlinearity in optical fiber dictates that very long lengths (e.g., 5-10 km) be used to obtain sufficiently efficient switching, even with the high peak power optical clock sources. This introduces a substantial delay (20-25 .mu.s) between the arrival and decoding of the data by the receive node.
Demultiplexing schemes based on optical nonlinearities in fiber or in semiconductors can demultiplex the full stream only by demultiplexig each of the channels in parallel. To accomplish this, copies of the full input stream must be split off, one for each base rate channel, and sent to separate identical demultiplexers tuned for the individual channel.